The special optical properties of nanosized semiconductors (quantum dots) have captured much imagination, promising a variety of applications in biology and medicine. Relevantly, the development of benign (nontoxic or less toxic) quantum dots alternative to those mostly containing heavy metals such as cadmium and lead is both scientifically significant and technologically valuable to many of the desired applications. We have recently found and reported that nanosized pure carbon particles may be surface-passivated to exhibit strong photoluminescence in the visible and near-infrared regions. The photoluminescent carbon particles, dubbed "carbon dots", require two conditions: one is that the particles must be quantum-sized;and the other is the necessary surface passivation by organic or biological species. Carbon as an element is benign, and its particles of various sizes have already been used in consumer products. Here we propose to further explore this novel configuration of carbon-based nanomaterials with interesting optical properties in two Specific Aims: (1) to improve the properties of carbon dots in reference to those of traditional semiconductor quantum dots. Our emphases for this Aim will be on the manipulation and control of the dot sizes and size distributions and on the selection of surface passivation, targeting (a) brighter photoluminescence for both one- and two-photon excitations;(b) the ability to obtain narrower emission bands;and (c) some understanding of the photoluminescence mechanism to aid further improvement of the optical properties. (2) To conjugate carbon dots with biological species and species with biological and/or biomedical significance, and to evaluate effects of the conjugation on optical properties of the carbon dots. Since the photoluminescence mechanism of carbon dots is likely different from that for traditional semiconductor quantum dots, effects of the conjugation on the optical properties of carbon dots will be evaluated and exploited for desired performance, which is a necessary step toward their eventual uses in vitro and in vivo. Collaborations with other researchers will be pursued and developed via sharing samples of most promising carbon dots to begin the toxicity and other biological evaluations.